This invention relates to hydrotreating processes and, in particular, to hydrodehalogenation processes and, more particularly, to hydrodehalogenation and deoxo processes.
Catalytic processes are used to promote, as well as enhance, the efficiency of various industrial processes, such as synthesis, conversion and/or fluid treatment processes. But one common weakness that many of these catalytic processes suffer from is a near-zero tolerance of halogenated hydrocarbons (halohydrocarbons) or carbon oxides that may be present in a gaseous feedstock to some processes. Consequently, the presence of small amounts of halohydrocarbons or oxides can lead to substantially increased process operating costs by premature reduction in catalyst activity (i.e., catalyst poisoning). Catalyst poisoning reduces process efficacy and efficiency, as well as increases the catalyst systems"" replacement frequency, which in turn, increase downtime and operating costs.
Some processes that have little to no tolerance to halohydrocarbons or oxides in the process feedstream include, without limitation, ammonia synthesis, hydrogenation (e.g., methyl acetylene and propadiene hydrogenation to propylene and propane), butadion (BDO) production, toluene diamine (TDA) production, hexamethyldiamine (HMDA) production, and hydrogen peroxide (H2O2) production. For example, halohydrocarbons often corrode equipment and/or poison catalysts, thereby reducing its catalytic activity. As another example, at low concentrations, sulfur, chlorine and halohydrocarbons can be poisons to catalysts used in the above-mentioned hydrogenation reaction and BDO, TDA, HMDA and H2O2 production. Also, sulfur, chlorine, halohydrocarbons and oxygen can be poisons at low concentrations to ammonia synthesis catalysts.
Accordingly, there has been a continuing effort to reduce or eliminate halohydrocarbons, present in various chemical process feedstocks, by converting halohydrocarbons to compounds that can be removed by conventional means or that do not have a deleterious effect on the catalysts.
Also, certain halogenated hydrocarbons, often called halohydrocarbons, have wide-ranging applications including use in adhesives, aerosols, various solvents, pharmaceuticals, dry cleaning textile processing and as reaction media. However, many halohydrocarbons, particularly fluorohydrocarbons and chlorohydrocarbons, can be toxic to human health and the environment at relatively low concentrations. In view of this potential toxicity, the use and environmentally acceptable emissions of many halohydrocarbons is becoming more stringently regulated in Europe, the United States, Canada and many other industrially developed communities. Accordingly, there have also been efforts to reduce or eliminate the halohydrocarbons by catalytically converting halohydrocarbons to less toxic or nontoxic compounds that have a reduced risk to health and the environment.
For example, in U.S. Pat. No. 4,039,623, Lavanish et al. disclose a hydrated nickel (Ni) oxide catalyst for lowering the C2 to C4 halohydrocarbon content in an oxygen-containing gaseous stream, such as an air stream. Lavanish et al. require that their hydrodehalogenation process be conducted at a temperature in the range from 20xc2x0 to 500xc2x0 C. and with a stoichiometric amount of oxygen (O2) sufficient for converting the carbon content to carbon dioxide. As well, hydrated nickel oxides having Ni in a +2, +3 or +4 oxidation state must be used for catalyzing the Lavanish hydrodehalogenation reaction.
Also, U.S. Pat. No. 5,021,383 and U.S. Pat. No. 5,114,692, both by Berty, disclose catalytically converting halohydrocarbons to nontoxic products using a catalyst composition having both a metal based catalyst and an alkali or alkali-earth carbonate, preferably with the catalyst dispersed in the carbonate. Berty discloses metal catalysts comprising a metal such as manganese, copper, silver, iron or aluminum or a metal oxide, such as nickel oxides, cobalt oxide, aluminum oxide, vanadium oxide, tungsten oxide, molybdenum oxide or mixtures thereof. The carbonate is required in Berty""s catalyst composition to react with hydrochloric acid (HCl) formed during the catalytic conversion process to prevent reformation of new halohydrocarbons. According to Berty, a carbonate, such as CaCO3, will react with HCl immediately, thereby preventing gaseous HCl from vaporizing and reoxidizing back to chlorine gas (Cl2), which can subsequently chlorinate another organic compound in the reaction feedstock. Thus, Berty believes a metal catalyst/carbonate composition is important to effectively hydrodehalogenating a feedstock.
As reported in the Journal of Catalysis, 74, 136-134 (1982), Weiss et al. studied hydrodechlorination and oligomerization of carbon tetrachloride (CCl4) using a nickel-based sodium Y zeolite (NiNaY) catalyst composition prereduced in a hydrogen (H2) atmosphere at 370xc2x0 or 530xc2x0 C. The reduced NiNaY catalyst was subsequently used to catalyze the reaction between H2 and CCl4 at 370xc2x0 C. Weiss et al. observed that NiNaY catalyzed reaction of H2 and CCl4 produced small amounts of methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10). To their surprise, however, they found that the NiNaY catalyst, most particularly a mixed nickel/cobalt (NiCo) Na Y zeolite, was most active and selective for producing predominantly 1,1,1,2-tetrachloroethane (Cl3CCH2Cl) per mole of CCl4 (i.e., 0.4 mole of Cl3CCH2Cl per mole of CCl4 at 80-100% conversion). And at lower CCl4 conversions chloroform (CHCl3) and hexachloroethane (C2Cl6) were also primary reaction products as well as Cl3CCH2Cl.
Weiss et al. concluded further, from X-ray photoelectron diffraction measurements, that xe2x80x9cin the case of the nickel-exchanged NaY catalyst, it is the nickel metal that is clearly the catalytic agent, the amount of Ni0 being a function of reduction temperature.xe2x80x9d Also, they hypothesized that xe2x80x9cthe zeolite environment is central to the tailoring of the reaction system [and] [n]ickel that has migrated out of the supercage behaves differently than Ni0 inside the supercage.xe2x80x9dAccordingly, Weiss et al. provided evidence that a Ni0 supported on a NaY zeolite can contribute to a hydrodehalogenation reaction, but with Cl3CCH2Cl, CHCl3 and C2Cl6 being the primary reaction products, among other halohydrocarbons. Moreover, Weiss et al. showed that NiNaY catalyst could produce only minor amounts of fully hydrogenated products, such as CH4, C2H6, C3H8 and C4H10, while predominantly producing halohydrocarbon products.
In U.S. Pat. No. 4,436,532, Yamaguchi et al. disclosed using a nickel-based, nickel/molybdenum-based (Ni/Mo) or cobalt/molybdenum-based (Co/Mo) catalyst in sulfided form to hydrodehalogenate gaseous feedstock, called pyrolysis gas, produced from pyrolyzing solid wastes at 550xc2x0 C. or greater. The pyrolysis gas is composed primarily of H2, carbon monoxide (CO), carbon dioxide (CO2), CH4, C2 and higher hydrocarbons, as well as smaller amounts of HCl, methyl chloride (CH3Cl) (i.e., about 1,000-1,500 ppm), ammonia (NH3), hydrogen sulfide (H2S), 100-1000 ppm of organosulfuric compounds, hydrogen cyanide (HCN) and trace amounts of other chlorohydrocarbons. Yamaguchi et al. observed that a non-sulfided hydrogenating catalyst would drive the methanation reaction (i.e., converting CO and CO2 into CH4). They also observed that this methanation reaction was undesired because it would produce xe2x80x9ctroubles,xe2x80x9d such as excessive temperature excursions due to high concentrations of CO and CO2, for the hydrodesulfurization (HDS) and hydrodehalogenation (HDH) reactions important to reducing the overall toxicity of the product stream. Accordingly, Yamaguchi et al. stressed the importance of using a Ni-, Ni/Mo- or Co/Mo-based catalyst in sulfided form to-concurrently promote both the HDS and HDH reactions, while at the same time suppressing the undesired methanation reaction (see col. 6, lines 62-68 of U.S. Pat. No. 4,436,532).
In U.S. Pat. No. 5,019,135, Sealock, Jr. et al. disclosed a biomass conversion process using a reduced Ni/alkali catalyst composition to convert mixtures of water and plant tissue containing lignin and at least 1 wt. % cellulose (i.e., lignocellulosic materials, such as sorghum, sunflower, potato waste, etc.) into a fuel gas composed primarily of CH4, H2 and CO2. This biomass conversion process is conducted in the temperature range of 300xc2x0 to 450xc2x0 C. and under a pressure of at least 100 atmospheres (i.e., 10 MPa or 1470 psi), to prevent water from boiling over in the reactor. Sealock, Jr. et al. determined that the alkali catalyst, selected from the group consisting of sodium, potassium or cesium ion, can be in the form of a carbonate, oxide or salt. And despite experimental results indicating that there is an inverse relationship between CH4 produced and the alkali carbonate concentration, they specified that at least 0.0001 mole of elemental alkali metal per gram of dry lignocellulosic material was required to produce an alkali/Ni co-catalyst system that could promote the biomass conversion process (see col. 10, lines 3-15 of U.S. Pat. No. 5,019,135). Also, there was no suggestion or experimental evidence that this co-catalyst system would facilitate hydrodehalogenation of halohydrocarbons.
As discussed above, Yamaguchi et al. observed, what is well understood by those skilled in art: a sulfided catalyst composition is required for catalytic performance in hydrodesulfurization (HDS) and hydrogenation (HYD) reactions, while a non-sulfided catalyst composition is required for catalytic performance in a methanation reaction (i.e., converting CO and/or CO2 into CH4). Also, in an exhaustive review and technical analysis of the literature on hydrotreating catalysis, compiled in Hydrotreating Catalysis by H. Topsoe, B. S. Clausen, F. E. Massoth, (1996, vol. 11, Catalysis, Science and Technology series), Topsoe et al. indicate it has been known for a long time that many transition metal sulfides are active catalysts in hydrotreating (see p. 208). Hydrotreating refers to a variety of catalytic processes which add H to unsaturated hydrocarbons while removing heteroatoms, such as S, N, O and metals to form saturated hydrocarbons with no heteroatoms. Accordingly, since CO and CO2 are not hydrocarbons, converting CO and CO2 to CH4 (i.e., methanation) is not typically viewed as hydrotreating type process. Thus, those skilled in the art of hydrotreating have typically used conventional sulfided catalyst compositions for (HYD) and, likewise, for hydrodehalogenation (HDH) processes.
However, it is difficult to maintain a sulfided catalyst in the presence of H2 and a low concentration of S-containing compounds (for example, 1-2 ppm (vol.)). Specifically, under these conditions, sulfur is stripped from the catalyst, releasing H2S, and metal is reduced to a free metal state. The catalyst thus becomes a hydrocracking catalyst, thereby promoting undesirable side reactions. Also, if a sulfided catalyst is used, a separate methanation catalyst reaction step is required, thereby making the process a two-step, rather than a one-step, process. Also, ultimate disposal of sulfur containing catalyst compositions can present additional environmental and safety issues often not encountered with non-sulfided catalyst systems. Moreover, two component catalyst systems, like those disclosed by Berty and Sealock, Jr. et al., requiring an additional non-metal component can be cumbersome to manufacture and/or use with consistent performance results. Also, the high pressure required to promote some conversion processes, such as Sealock, Jr. et al.""s biomass conversion process, increases the operating costs and introduces a potential safety risk that must be managed.
Accordingly, there is a need for a non-sulfided catalyst system that can dehalogenate substantially all halohydrocarbons in a gaseous feedstock to saturated hydrocarbons, preferably without the requirement for an additional process step and, more preferably at pressures below 10 MPa (1470 psi).
According to the invention, there is provided a process for treating a gas stream having at least about 90 volume percent of H2, based on the total volume of all constituents comprising said gas stream, at least one halogen-containing compound and a total concentration of S-containing compounds less than about 2 ppm by volume, based on a mono-sulfur compound equivalent, said method comprising:
(a) contacting said gas stream with a nickel catalyst composition, wherein said halogen-containing compound contacts said nickel catalyst composition so that a substantial portion of said halogen-containing compound is reduced, said catalyst composition comprising a source for nickel in a zero oxidation state, Ni0, selected from the group consisting of
(i) at least about 5 weight percent Ni0,
(ii) a Ni0 precursor having Ni+n, where 1xe2x89xa6nxe2x89xa64, that can produce at least about 5 weight percent Ni0 under substantially reducing conditions, and
(iii) combinations thereof,
said Ni0 weight percent being measured as a percentage of the total weight of all constituents comprising said nickel catalyst composition; and
(b) producing a nickel catalyst-treated gas stream, wherein substantially all halogen-containing compounds are de-halogenated.